Exploration On New Nb-based Superconductors And Critical Behavior Of Anti-perovskite Mn₃GaC

In this thesis,we prepared a series of Nb-based materials by solid-phase reaction experimental methods and explored their superconducting properties.We used VESTA software to draw their crystal structure diagram,and analyzed the relationship between the diffraction peak of the material and the diffraction angle by X-ray diffraction?XRD?method.Next,their XRD data were fitted by crystal repair fitting with FULLPROF software,so the lattice parameters a,b,c,lattice volume,phase content ratio and other information can be obtained.Then we carried out a measurement of their electrical transmission through physical property measurement system?PPMS?within a specific temperature range?2-300K?to determine whether there is superconductivity.So,we would will continue to measure the magnetic properties through magnetic measurement system?MPMS?within the same temperature range,and analyze its superconducting characteristics through electrical transport and magnetic measurement.We also studied the properties of a series of Ga_1-xCMn_3+xmaterials.Starting from the preparation and research of several different materials,we obtains the following research results:1. We prepared a series of samples of Nb₅Ge₃ precursor and doped materials by the solid-phase reaction experiment method.From the crystal repair results of XRD data,information such as lattice parameters a&b and c,lattice volume,and content ratio of two different phases were obtained.After analysis,the relationship between the series of Nb₅Ge₃ precursors and doped materials with the content of C or B atoms or quenching temperature is obtained.From the electrical transmission test,we found that different Nb₅Ge₃ precursor and doped materials showed metal characteristics at high temperature,but there is no sudden drop of resistivity representing the superconducting phenomenon at the low temperature.The sample completely dropped to zero resistance state?ie Nb₅Ge₃C₁?,and the superconductivity of the sample was analyzed and judged to be caused by a small amount of impurities present inside.Moreover,the electrical transport of different Nb₅Ge₃ precursors and doped samples is normalized.We compared the changes of resistivity of doped samples with different contents with figures.We found that under 10Oe field,most samples exhibited paramagnetic behavior,while the doped samples of Nb₅Ge₃C_0.5 and Nb₅Ge₃C₁ exhibited diamagnetic behavior with superconducting properties,by using magnetic susceptibility measurement.Therefore,we calculated the integral number of superconductors for the samples with these two components,and found that both integral numbers are less than 1%,thus concluding that this superconducting phenomenon is caused by a small amount of internal impurities.2. We prepared a series of NbB_2+x samples by the solid-phase reaction experimental method.Through XRD analysis,the relationship between the diffraction peak position of the material and the doping is obtained.Reliable lattice parameters a&b and c,lattice volume and other information are obtained from the crystal repair fitting data of XRD data.It can be seen that different Nb B_2+x present metal features at high temperature from the test results of electrical transmission,while their resistivities drop by different degrees at low temperature,but they did not completely become the zero resistance state(except Nb B_2.1),probably due to the incomplete superconductivity caused by a small amount of impurities in the series of samples.It was found that most samples exhibited diamagnetic behavior with superconducting characteristic at low temperature,and only Nb B_1.98 exhibited paramagnetic behavior,from the magnetic susceptibility measurement.We conducted a study on the Nb B_1.98 component sample under high-pressure conditions.We can also clearly see that as the pressure increases,the resistivity of the material gradually decreases from the high-pressure test results of the Nb B_1.98 material.It can be seen that the relative resistivity of?/?_5K in the low temperature range increases with pressure,and the corresponding transition temperature first increases and then decreases,from the normalized resistivity results.3. We obtained a series of Ga_1-xCMn_3+xmaterials through the solid-phase reaction experiment method.The crystal repair fitting was applied to their XRD data using FULLPROF software.Reliable lattice parameters a&b&c and unit cell volume of the polycrystalline sample can be obtained from the results of these crystal repairs.We also obtained a high-purity Ga CMn₃ polycrystalline sample,confirmed by analyzing through JADE6 and crystal repair software.The electrical transmission of this Ga CMn₃material was tested in a specific temperature range?1.8-400K?without magnetic field.We can see from the measurement results that the resistivity of the sample is gradually decreases as the temperature decreases within the measurable range,and there are a slope increase at 250K,and a sudden jump near 155K.The resistivity decreases again as the temperature further decreases.The change in resistivity with temperature is consistent with the AFM-FM transition,except for the presence of small hysteresis.We analyzed the magnetization M and susceptibility?^-1 of the Ga CMn₃ sample under different field strengths?0.01T,0.1T and 1T?under zero field cooling?ZFC?and found that the magnetic transition temperature T_c increased with the applied magnetic field H.It shifted to a high temperature area and increased by about 10K.The susceptibility?^-1curve can be fitted well by Curie-Weiss law,which is basically consistent with previous reports.Then we analyzed the measurement results of the magnetization M of a series of Ga_1-xCMn_3+x?x=0,0.06,0.07,0.08?samples during zero-field cooling?ZFC?under1T external magnetic field.We can see that the slope of M?T?decreases as x increases and becomes negative.The absolute value of d M/d T is the largest at Ga CMn₃,reaching1.2 emu/g·K in the zero-field cooling process under the external magnetic field of 1T.And as x increases,the peak height decreases,while peak width increases,and the magnetic transition temperature T_c shifts toward the high temperature region.Finally,we explained the origin of the magnetic heating effect?MCE?near the magnetic phase transition of the FM-PM of the Ga_1-xCMn_3+x material through the Landau phase transition theory,molecular average field model and specific heat measurement.We found the Landau phase transition theory only considers magnetoelastic coupling and electronic interaction but not electro-phonon coupling,so it can not explain the behavior of magnetic entropy change?-?S_M?,but the molecular average field model and specific heat measurement can explain the existence of Magnetic entropy change?-?S_M?behavior.In order to understand the FM-PM phase transition properties of this series of Ga_1-xCMn_3+x materials,we studied the critical behavior near the phase transition by Modified Arrott plot,Kouvel-Fisher plot and Critical Isotherm analysis methods,and introduced the mean field model and three-dimensional Heisenberg model to study its critical index and found that the critical index value we obtained is closer to the mean field model result.